/Engine/Source/Runtime/SlateCore/Public/Layout/Children.h
For widgets that do not own their content, but are responsible for presenting someone else's content. e.g. Tooltips are just presented by the owner window; not actually owned by it. They can go away at any time and then they'll just stop being shown.
template<typename ChildType> class TWeakChild : public [FChildren](API\Runtime\SlateCore\Layout\FChildren)